1. Field of the Invention
The present invention relates to a method and an apparatus for producing an oxide glass thin film to be used for formation of optical waveguide or the like.
2. Related Background Art
A glass optical waveguide formed on a silica glass substrate or on a silicon substrate shows good matching with optical fibers, and, therefore, is used as a component for optical communication. There is a method for forming a glass optical waveguide on a substrate, in which fine particles of glass are deposited on a substrate by a gas phase method using an oxygen-hydrogen burner to form a porous glass thin film thereon (see FIG. 1) and the substrate is then heated at a high temperature by means of an electric oven to finally form a transparent glass thin film on the substrate. Normally, the substrate is a silicon wafer, and the waveguide as so formed contains a main component of SiO.sub.2. In this case, the melting point of silicon is lower than the glass transition temperature of pure SiO.sub.2. It is a usual practice to lower the glass transition temperature by adding an additive or additives such as P.sub.2 O.sub.5, B.sub.2 O.sub.3, GeO.sub.2, and the like into SiO.sub.2. The specific contents of the conventional example are described in detail for example in Japanese Laid-open (Kokai) Patent Application No. 58-105111, Japanese Laid-open (Kokai) Patent Application No. 1-192732, and U. S. Pat. No. 4,263,031.
In case that the fine particles of glass are deposited on the substrate to form the porous thin film and heated at the high temperature by the electric oven to form the transparent glass thin film, as described above, the following problems would occur by volatilization scattering of additive components added into the oxide glass thin film in order to lower the glass transition temperature.
Components easy to volatilize to scatter, such as P.sub.2 O.sub.5, B.sub.2 O.sub.3, etc., among the additive components first go out of the thin film, which raises the glass transition temperature of the glass portion from which the volatile components have been volatilized to scatter. The rise of the glass transition temperature results in leaving an unsintered portion or non-transparent glass portion behind in the glass film produced. This would cause a high optical loss in a waveguide thus formed from such a glass film. Additionally, the refractive index of the glass film surface varies depending upon the volatilization of the additive components, which makes it difficult to obtain a desired arrangement of refractive index. This in turn makes the design of waveguide structure difficult.